These are photos of the Onbag Mk1 - I used black cotton drill and very cheap lining fabric to mock it up but I think it worked surprisingly well. Here you can see me modeling it with little Charlie in the sling.
First as a shoulder bag (rebozo style) and then as a rucksack - I could really do with wearing it a little higher though!
Although this one was really only a tester I used it for several months before I got round to making myself a more respectable version and it held up really well.
I've found the bag to be comfortable worn either way - and the fact that different friends wear their's different ways suggests that the various options do work...